Use MG_EV_WS_CTL for WS control messages

src/util.h

@@ -17,7 +17,7 @@ void mg_unhex(const char *buf, int len, unsigned char *to);
 unsigned long mg_unhexn(const char *s, int len);
 int mg_asprintf(char **buf, size_t size, const char *fmt, ...);
 int mg_vasprintf(char **buf, size_t size, const char *fmt, va_list ap);
-int64_t mg_to64(const char *s);
+int64_t mg_to64(struct mg_str str);
 double mg_time(void);
 unsigned long mg_millis(void);
 void mg_usleep(unsigned long usecs);